(353f) Droplet Dynamics in Channel Flow with Sudden Expansion

Drop Dynamics in a Sudden Expansion

Robert M. Carroll and Nivedita R. Gupta

Department of Chemical Engineering

University of New Hampshire

Durham, NH 03824

Abstract

Drops in channels of varying cross-section are encountered in a variety of applications.  To understand the behavior of drops as they translate through a sudden expansion, we conduct a numerical study using a three-dimensional axisymmetric hybrid volume-of-fluid method combined with a front-tracking scheme.  The simulations begin with a drop in the small channel at its expected steady shape.  As it moves through the expansion, the drop elongates perpendicular to the direction of flow taking on an oblate shape.  Further away from the expansion region, the drop reverts back to the steady shape predicted for the larger channel size.  We study the dynamics of this process at a low but finite Reynolds number and a variety of drop sizes, capillary numbers, and channel expansion ratios.  We specifically collect data on maximum deformation, drop speed relative to the fluid, and changes in drop surface area. For large drops, bullet- and parachute-shaped drops as well as drops with a negative curvature are observed.  We validate the numerical simulations by comparing the results with experimental studies of glycerol-water drops in silicon oil flowing through a PDMS channel.  We extend the numerical results to study the effect of soluble surfactants in the adsorption-desorption limit.  The surfactants are modeled using a Langmuir adsorption framework.  The non-uniform distribution of surfactants at the interface during the deformation process gives rise to Marangoni stresses that affect the maximum deformation and drop surface area.